The present invention relates generally to electrical generators, and more particularly to a system for monitoring the vibration of an electrical generator.
U.S. Pat. No. 5,146,776, Sep. 15, 1992, titled Method for Continuously Calibrating an Optical Vibration Sensor, discloses a system for automatically calibrating a fiber optic vibration monitor (FOVM) employing a cantilever-mounted grid attached to a generator. The grid interrupts a light beam at a frequency directly proportional to the sensor's vibrational amplitude at a singular driving frequency (i.e., 120 Hz). The system disclosed in the patent is illustrated in FIG. 1. A generator 10, optical vibration sensor 12, and computer 14 constitute the vibration sensing system 16. The patent teaches how troublesome conditions of the generator can be detected at an early stage by measuring the vibration amplitude of a generator end-winding. This allows maintenance to be scheduled to avoid damage to the generator and minimize down time.
Briefly, the system may be described as follows: The optical vibration sensor 12 is mounted directly to an end-winding 17 of the generator 10. The massive exciter-end and turbine-end end-turns of the generator are consolidated into semi-ridged baskets to prevent damaging effects of the 120 Hz vibration coupled into the system from the rotor field. The sensor monitors the end-turn vibration to provide warning signals when destructive levels of vibration exist or when the vibration level is increasing. The vibration may then be controlled through load management or change in coolant gas temperature until an outage can be scheduled for the generator.
FIG. 2 illustrates the optical vibration sensor 12 in more detail. The optical vibration sensor 12 receives light from an optical fiber cable 18. The sensor includes a housing 20 and an optical-to-digital conversion unit 22. The housing 20 includes an internal reed 24 and a grid assembly 26. The internal reed 24 and the grid assembly 26 are designed to have a natural resonant frequency above 120 Hz. Preferably, the resonant frequency is approximately 132 Hz for a 60 Hz generator application. See U.S. Pat. No. 4,321,464, Mar. 23, 1982, or U.S. Pat. No. 4,218,614, Aug. 19, 1980, for further details of the sensor 12.
The following discussion assumes the generator is producing 60 Hz electrical power, although the principles are the same for a 50 Hz unit.
As the internal reed vibrates, the grid assembly 26 moves up and down, causing light pulses to be produced. The number of light pulses produced in a given time interval is proportional to the amplitude of the 120 Hz (100 Hz in Europe) vibration being measured. The grid assembly 26 has evenly spaced grid openings separated by 10 mils. Thus, the number of light pulses produced in a given time interval is a function of the resonant frequency of the sensor and the distance the grid swings from its equilibrium position. The light pulses are output from the casing 20 through the optical fiber cable 18 to the optical-to-electrical conversion unit 22. The optical-to-electrical conversion unit 22 converts the light pulses into a digital signal according to a conventional method. For example, a photodiode can be utilized to convert the light pulses to an electrical signal which can then be converted into a digital frequency output signal. The output signal waveform takes the form of a frequency modulated sine wave. The signal is, furthermore, slightly frequency-modulated by the mixing of the 120 Hz excitation with the resonant frequency of the sensor.
The system employs curve fitting of the beat signal peaks to a trigonometric function of the form sin(2.pi.f.sub.B t) to determine the beat frequency f.sub.B. The beat frequency is then used to calibrate the system. In particular, the system computes an amplification factor EQU M.sub.0 =(120/f.sub.0).sup.2 /(1-(120/f.sub.0).sup.2),
where the sensor's resonant frequency is given by EQU f.sub.0 =120 Hz+f.sub.B.
Thus, the resonant frequency f.sub.0 of the optical vibration sensor determines the amplification factor M.sub.0. To obtain the actual displacement of the generator due to vibration at 120 Hz, the measured amplitude (i.e., as determined by the light pulse signal) must be divided by the amplification factor. Note that the equation for M.sub.0 results from the correlation between the light pulse frequency and the amplitude of the grid, which can be expressed, for the grid-reed geometry employed by the assignee (Westinghouse), as:
Amplitude of vibration=f.sub.LP .times.1 mil/180 Hz, where f.sub.LP is the light pulse frequency (Hz). This equation is true for a grid assembly having a grid spacing of 10 mils.
In sum, the system employs the amplitude of the signal at the "extrema" to determine the beat frequency. Such a beat frequency is discernable from FIG. 3A, which depicts a waveform representative of an ordinary sensor signal. The extrema are the furthest points in the grid's motion as it oscillates about its equilibrium position. The largest wavelengths in the frequency modulated output signal (i.e., the points in the waveform where the zero crossings are spread apart the most) correspond to the extrema, since the extrema are where the grid comes momentarily to rest before reversing direction.
The present invention addresses the problem that occurs when the beat amplitude becomes large enough to cause a fold-over, distorting the beat frequency. This problem also occurs in connection with a small beat amplitude when the signal at the extrema occurs near the peak signal values. An illustration of a small fold-over phenomenon is shown in FIG. 3B. Very large fold-overs often occur in the field. However, the waveform extrema for such large fold-overs are difficult to visualize and thus are not depicted. As discussed above, to determine the actual displacement of the generator due to vibration at 120 Hz, the amplification factor M.sub.0 must be determined. To determine the amplification factor, the resonant frequency of the sensor (f.sub.0 +f.sub.B) must be accurately determined. However, when fold-overs occur, they distort the beat signal determined from the extrema such that it becomes extremely difficult to determine the beat frequency, making it practically impossible to accurately determine the resonant frequency of the sensor. Moreover, the resonant frequency drifts (changes) with temperature and with age of the sensor. Therefore, one cannot assume that the resonant frequency of the sensor is whatever it was designed to be. It must be measured in the field, while the generator is operating.